The present invention relates to the field of satellite communications. More particularly, the present invention relates to the field of power monitoring and control for satellite transponders.
A communications satellite typically operates by receiving communication signals modulated according to individual carrier frequencies from ground-based transmitters. A transponder in the satellite typically processes the signals before re-transmitting them. For example, the transponder typically amplifies and may also translate the received signals in frequency and perform filtering prior to retransmission. Ground-based receivers may then receive the re-transmitted signals.
A number of competing factors may affect the level of power used for transmission of satellite communication signals. For example, weather patterns can interfere with satellite communications. More particularly, rain can reduce the effective power of the communication signals. This is often referred to as xe2x80x9crain fade.xe2x80x9d Thus, in the presence of rain, signal power levels are generally required to be higher than otherwise to ensure that the signals are received reliably.
A typical satellite may have up to twenty-four or more transponders, while each transponder may handle up to one hundred or more carrier signals. Depending upon communication demands, not all of the carrier signals may be active at any one time. Thus, total power requirements can vary depending upon communication traffic levels. However, the total power available for retransmission is limited. For example, the on-board power supplies for the satellite have limited output.
In addition, the transponder amplifiers used for retransmission of the carrier signals have limited output power. More particularly, at low power levels, an amplifier may be operating in a region in which there is a substantially linear relationship between input signal power and output signal power. At higher power levels, the relationship between input signal power and output signal power becomes increasingly non-linear. This gain characteristic is illustrated in FIG. 1. As can be seen from FIG. 1, at low output power levels (e.g., below approximately 15-20 watts), the gain response is relatively linear. At higher power levels, the gain response becomes compressed. Some compression may be tolerable. Operation beyond certain levels of compression, however, is generally undesirable as the resulting signals can become corrupted, such as with excessive noise. A position on the gain characteristic at which an amplifier is operating is often referred to as its xe2x80x9coperating point.xe2x80x9d
Thus, it is often desired to determine the operating point of a transponder amplifier in a satellite. A conventional technique involves measuring the power level of a communication signal received from the satellite at a ground station. Then, by estimating the signal attenuation between the satellite and the ground station, the power level at the satellite may be estimated. This estimated power level indicates the then-current operating point. This technique has a drawback in that the estimation of attenuation is prone to error. As such, the operating point cannot be determined with accuracy.
Accordingly, there is a need for a more accurate technique for determining the operating point of a satellite transponder amplifier. Further, it may be desired to control the power level of the amplifier in response to the measured operating point so as to ensure that the retransmitted signals are not overly compressed. It is to these ends that the present invention is directed.
The invention is a method and apparatus for monitoring and controlling the operating point of a satellite transponder amplifier. In one aspect of the invention, a time domain signal received at a ground station is digitally sampled. Then, a histogram of the received signal is computed. To form the histogram, the digital samples are categorized according to which of a plurality of amplitude ranges each sample falls within. When plotted as a graph, the number of occurrences within each amplitude range forms the histogram. Under low compression levels, the histogram generally appears as an approximately bell-shaped curve. However, under higher compression levels, the bell-shaped curve becomes distorted. By correlating the acquired histogram to a template histogram, an amount of compression can be determined. This amount of compression indicates the operating point of the transponder amplifiers. If desired, the transmission power levels used by the satellite can be adjusted to ensure that the amplifiers operate at desired levels of compression. The invention has an advantage over prior techniques in that the operating point of the transponder amplifiers can be more accurately determined. As such, the operating point can be more precisely adjusted.